1. Field of the Invention
The present invention relates to a toner set, a developer set, and an image forming apparatus.
2. Description of the Related Art
For an electrophotographic image forming apparatus of recent years, there have been increasing demands for saving of energy during toner fixing and for an image forming apparatus capable of high-speed processing, and a toner itself is required to have a property to melt at a low temperature. There are also large demands for high-quality image, and for a demand of high-definition image formation such as photographic image, it has been known that a vivid high-gloss image may be provided by imparting gloss on a surface of a recording medium such as recording paper.
For example, a method of disposing a transparent toner in a non-image area without a color toner is disclosed (see Japanese Patent Application Laid Open (JP-A) No. 04-278967). In this method, in electrophotography used in an image forming apparatus such as recording laser printer, dry electrostatic copier and so on, a surface of the image bearing member such as photoconductive layer is uniformly charged. Subsequently, the surface of the image bearing member is exposed, and gloss differences between an area with a color toner and an area without a color toner of exposed areas on a medium, or a transparent toner is disposed on a whole surface of a recording medium.
However, a method for manufacturing the toner includes a step for polymerizing by polyaddition reaction of a polyester prepolymer having an isocyanate group with an amine in a reaction system in which an organic solvent and an aqueous medium co-exist. In a case of this method and a toner obtained by this method, high temperature-resistant offset property improves, but there are problems of inhibiting low-temperature fixing property and reducing gloss after fixing.
Also, a method first to print and fix a gloss area and second to print and fix a non-gloss area is proposed (see JP-A No. 04-338984). It is possible to obtain different glosses on an identical recording medium according to these methods.
In this method, to improve low-temperature fixing property, crystalline polyester is introduced to a toner obtained by a polymerization method. However, a dispersion having a small particle diameter may not be obtained in a stable manner, and as a result, there is a problem that toner particle size distribution degrades.
Also, as an electrophotographic method for forming different glosses on an identical recording medium, a method to control gloss by a number-average molecular weight of a resin used for a toner is proposed (see JP-A No. 08-220821). In this method, a polyester resin having a number-average molecular weight of about 3,500 is used as a transparent toner, and a polyester resin having a number-average molecular weight of about 10,000 is used as a color toner. Because of the transparent toner having a melting point lower than the color toner, smoothness increases, and glossiness of a portion of the transparent toner becomes partially high.
However, the transparent toner is formed at a top of an image, and it contacts directly with a fixing machine. Thus, it is required to have higher hot-offset resistance than the color toner. At the same time, since the transparent toner is formed on a color-toner image, a toner layer becomes thick. There is a problem that color toner lacks stability unless it has high cold offset property.
Also, a method of forming a transparent toner image after fixing a color toner and reducing a fixing temperature so as to decrease gloss is proposed (see JP-A No. 2009-109926). However, in this method, a melt viscosity of the toner reached in a fixing nip during second image formation is configured higher than that in the fixing nip during a first image formation. Therefore, there is a problem that gloss falls because the transparent toner image prepared during the second image formation does not sufficiently melt.